Complex gear trains often use ring gears having internal teeth. Some of these gear trains, such as those used in automotive transmissions and the like, advantageously use helical gears rather than straight gears even though helical gear teeth are more difficult to form. Additionally, in many of these instances the internal gear teeth must be formed with very precise dimensions and spacing in order to perform adequately. Consequently the need arises to be able to fabricate ring gears having internal helical teeth that are precisely formed.
One method for precisely forming helical teeth is broaching, which is a cutting process. In broaching, a large broaching bar with cutting teeth is pulled through a gear blank to form the teeth. Broaching has drawbacks, however, in that it is an expensive process which requires a significant investment in expensive machinery and cutting tools. For example, a broaching bar that is used to form an internal helical ring gear for an automotive transmission may have to be as much as eight feet long, which is expensive to fabricate. Further, broaching can only be used on "through" parts since the long broach bar must be pulled through the inside of the gear blank to cut the teeth. Broaching, therefore, cannot be used at all to fabricate a blind end gear.
Gear shaping is another cutting process that can be used to fabricate internal helical teeth. Although it is a slower process than broaching, it can be used to form blind end as well as through parts for high volume production. Even so, this process also requires an investment in expensive machinery and cutting tools.
A process for the forming of internal helical gear teeth that is faster than shaping and broaching and requires less expensive tooling is cold extrusion. Cold extrusion is a process where teeth are formed into a part rather than cut into a part. A process for extruding internal teeth in a ring gear is disclosed in U.S. Pat. No. 4,878,370 to Fuhrman et al. The process disclosed therein is a two step process in which an annular work piece is advanced part of the way across external die teeth of a floating mandrel, and then a second work piece is inserted and used to push the first work piece through as the second one begins to be formed. Since each succeeding work piece is used to push the preceding work piece through, this process cannot produce blind end parts. Further, if helical teeth are being formed with the process disclosed in this patent, there is no external helical guidance while the teeth are being formed; only the helix of the die teeth are used to cause helical rotation of the work piece. This type of directional rotation will cause the amount of force that a hydraulic press must apply to extrude the work piece around the die teeth to increase since large friction forces will occur as the work piece slides along the annular inner surface of the die ring.
The need arises, then, when one desires to precisely form internal helical teeth in a blind end part to be able to extrude the gear teeth in a cost efficient manner.